UHF electronic tuner

ABSTRACT

A high input selectivity UHF varactor tuner for a television receiver in which unamplified radio frequency signals are inductively coupled to a double tuned half-wave preselector comprising two aperture coupled half-wave tuned lines in separate resonant cavities. The double tuned preselector includes varactor diodes on one end of each tuned line and fixed capacitors on the other end of each line. To compensate for decreasing Q of the varactor diodes with decreasing frequency and thereby maintain a uniform bandwidth for the tuner over the entire UHF spectrum, inductive coupling is provided between the tuned lines at the fixed capacitor ends which, during low frequency operation of the tuner, augments aperture coupling between the lines.

BACKGROUND OF THE INVENTION

The present invention relates to a UHF tuner employing varactor diodesand half-wave tuned lines in a manner which results in a substantiallyuniform bandwidth over the entire UHF spectrum.

Conventional UHF tuners for use in commercially available televisionreceivers comprise several stages: a broadly single tuned preselector, aradio frequency preamplifier to amplify the broadly tuned incoming radiofrequency signals, a double tuned interstage following the preamplifiertuned to peak at one select radio frequency signal, an oscillator togenerate an oscillator signal, and a mixer to heterodyne the oscillatorsignal with the select radio frequency signal to result in anintermediate frequency signal. The resultant intermediate frequencysignal is usually applied to the VHF turner and from the VHF tuner thesignal is fed into the intermediate stage of a television receiver forfurther processing in the same manner all VHF signals are processed forreduction to video and audio information.

The radio frequency preamplifiers of the prior art UHF tuners are proneto poor input selectivity resulting in a tendency to cross modulate. Inaddition, automatic gain control in UHF radio frequency preamplifiersvaries considerably with frequency resulting in a limited dynamic rangeor signal handling capability.

The preselector of prior art UHF tuners typically employs some form ofresonant cavity which, in essence, is a space enclosed by conductingwalls. The cavity has a resonant frequency defined by the modes ofelectrical fields which can be sustained in that space. The fundamentalresonant frequency of a cavity is thus determined by the physicaldimensions and configurations of the cavity's walls which act, ineffect, as the outer conductor of a coaxial line. In the alternative,substrates and strip lines may be employed to establish the fundamentalresonant frequency of the preselector.

Power is coupled to a tuned line mounted inside the cavity in either aquarter-wave (λ/4) or half-wave (λ/2) tuning mode. In the quarter-wavemode, one end of the tuned line is shorted directly to the cavity wallswhile the other end of the tuned line is capacitively coupled to thecavity walls. Accordingly, a null or low impedance point develops at theshorted end and a peak or high impedance point develops at the other endof the tuned line forming a quarter-wave standing wave. In a half-wavemode, both ends of the tuned line are capacitively coupled to the cavitywalls resulting in peaks being developed at each end of the tuned lineand a null or low impedance point developed at the node intermediate theends of the tuned line thereby forming a half-wave standing wave. In thesame manner, quarter-wave and half-wave tuning can be established inpreselectors using strip line design.

In any case, the resonant frequency of the line is controlled or tunedby varying the capacitance used to couple the tuned line to the cavitywalls. With quarter-wave tuning, varying the value of the capacitancevaries the resonant frequency of the line, however, the high impedancepoint remains fixed at the end of the line capacitively coupled to thecavity walls. With half-waves tuning, varying the value of thecapacitance also varies the resonant frequency of the line, but the nodeshifts along the line dependent on the relative values of the capacitorsat each end of the line. Furthermore, a higher impedance point isdeveloped at the lower capacitance of the two capacitors.

There are two fundamental types of variable capacitors used to vary thefrequency of tuned lines in UHF tuners. The first type is a mechanical,air dielectric, design wherein the position of metal plates is alteredrelative to each other to vary the capacitance of the device. The secondtype is an electric, varactor diode, design wherein the capacitance ofthe diode varies inversely with the magnitude of reverse bias placedacross the diode.

The use of varactor diodes in UHF tuners has a major advantage overmechanical capacitors in that varactor diodes permit UHF stations to beselected easily by means of an electric rotary switch or push buttons.Varactor diodes also render easy tuning to UHF stations by remotecontrol.

There is, however, a major disadvantage in the use of varactor diodes inUHF tuners. Specifically, a varactor diode exhibits a decreasing Q, afigure of merit of an electrical circuit, as the capacitance of thevaractor diode increases. Thus, when used in a tuned circuit of a UHFtuner, varactor diodes cause a decrease in the Q and consequently adecrease in the overall quality of the tuner at the low end of the UHFfrequency spectrum when the capacitance of the varactor diode is at amaximum. The decrease of Q in a tuned circuit has the specificdisadvantage of increasing the bandwidth of that tuned circuitproportional to the decrease in the Q of the circuit.

To compensate for the varying Q factor, a tuned circuit using a varactordiode can be damped, for example, by utilizing a frequency-dependentresistor to load a tuned circuit at the high end of a frequency range,as is taught by U.S. Pat. No. 3,573,683 issued to Marshall et al on Apr.6, 1971. However, commercially acceptable UHF tuners employing varactordiodes generally compensate for the decreased Q and resultant overallloss in the tuned circuits at low frequencies by providing a radiofrequency preamplifier in front of the UHF double tuned interstage toassure sufficient signal strength over the entire UHF frequencyspectrum, but even so, in such tuners there is no compensation forlosses incurred in the input preselector stage.

For quarter-wave tuners, the preamplifier can be an FET variety whichhas a high impedance output and can readily be coupled to thenon-shifting, high impedance end of quarter-wave tuned line. However,the state of the art UHF FET devices at present exhibit limited gaincontrol capability and as a consequence cause overload problems in highgain tuners. For half-wave tuners, the preamplifier can be a bipolarvariety which has a low output impedance and can be coupled at a fixedcapacitor of a half-wave tuned line which over at least a portion of thefrequency spectrum exhibits a low impedance. However, in the case of abipolar gain controlled preamplifier, radio frequency cross modulationcan become a problem. In both cases, the radio frequency amplifiersexhibit shortcomings which force compromise in various performanceparameters and require additional expense including the need forinterface components.

The present invention overcomes these disadvantages of the prior art byemploying a double tuned preselector circuit in front of the firstnon-linear device to achieve optimum UHF tuner selectivity. The doubletuned preselctor circuit configuration compensates for the varying Qfactor in a manner which allows for large signal handling capabilitywith no UHF radio frequency preamplifier. Instead, a post intermediatefrequency gain controlled amplifier allows for frequency independentgain control operation in which the VHF tuner and UHF tuner automaticgain control signals can be derived from the same source withoutinterface.

It is accordingly an object of the present invention to provide anoptimum input selectivity UHF varactor tuner with a substantiallyconstant bandwidth over the entire UHF spectrum.

It is another object of the present invention to provide a UHF tunerusing varactor diodes which exhibits a uniform bandwidth across theentire UHF spectrum.

It is a further object of the present invention to provide a UHFvaractor tuner with a constant bandwidth across the entire UHF spectrum,and which does not require radio frequency preamplification of anincoming UHF signal.

A still further object of the present invention is to provide a UHFvaractor tuner with large dynamic range and automatic gain controlindependent of UHF signal level and frequency selection.

SUMMARY OF THE INVENTION

To achieve the foregoing objects and in accordance with the purposes ofthe invention, as embodied and broadly described herein, the UHF tunerof this invention comprises a preselector for passing a select radiofrequency signal which has a plurality of tuned transmission lines and aplurality of varactor diodes for providing variable half-wave tuning ofthe transmission lines. An oscillator generates an oscillator signal anda mixer is coupled to heterodyne the oscillator signal with the selectradio frequency signal to result in an intermediate frequency signal. Inaddition, a circuit is provided to enhance radio frequency couplingbetween the tuned transmission lines at lower frequencies of the UHFspectrum and thereby compensate for decreasing Q of the circuit causedby low frequency, high capacitance, operation of the varactor diodes.

In a preferred embodiment of the invention, the preselector includes twohalf-wave transmission lines each housed in an individual one of firstand second preselector resonant cavities with each transmission linecoupled at one end to the walls of its cavity by a capacitor and at theother end each transmission line is coupled to the walls of its cavityby a varactor diode. Aperture coupling is provided between the twopreselector cavities and reverse bias is applied to the varactor diodesto control the capacitance of the varactor diodes and thereby doubletune the preselector to a desired resonant frequency. In this embodimentof the invention, inductive coupling is provided between the two ends ofthe transmission lines which are connected to the capacitors. During lowfrequency operation, the capacitance of the capacitors is much less thanthe capacitance of the varactor diodes causing the higher impedancepoint of the half-wave lines to develop at the capacitors. Thisdevelopment of high impedance at the capacitors both minimizes theeffect of the decreasing Q of the varactor diodes on the overall circuitand renders effective the inductive coupling between the transmissionlines to augment the aperture coupling between the lines and maintain asubstantially constant bandwidth and high Q over the entire UHFspectrum.

It is also preferred that unamplified radio frequency signals from a UHFantenna are coupled to the double tuned lines of the preselector toprovide optimum input selectivity in front of the first non-lineardevice of the tuner. In addition, a low noise post intermediatefrequency gain control amplifier is coupled to control the magnitude ofthe intermediate frequency signal from the mixer for application of thecontrolled intermediate frequency signal to an intermediate frequencysignal input of a VHF tuner.

DESCRIPTION OF THE DRAWING

A greater appreciation of the objects and advantages of the inventionmay be understood by a detailed description taken in conjunction withthe drawing wherein a schematic diagram of a preferred embodiment of theinvention is illustrated.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferred embodimentof the invention, an example of which is illustrated in the accompanyingdrawing.

Referring now to the drawing, a double tuned half-wave preselector stageof a varactor tuner is shown in accordance with the present invention ascomprising first and second tuned transmission lines 10 and 12 mountedin first and second resonant cavities 14 and 16 defined by conductivewalls of a housing 18. Tuned transmission lines 10 and 12 may, forexample, comprise bars, strips, wires or other conductive materialscapable of setting up standing waves in the UHF radio frequencyspectrum. Transmission line 10 is connected to resonant cavity 14 in ahalf-wave mode with both ends capacitively coupled to cavity 14.Specifically, one end of transmission line 10 is coupled to the walls ofcavity 14 by a fixed capacitor 20. Fixed capacitor 20 is preferably of afeed-through variety which allows for DC coupling to transmission line10 while simultaneously providing AC coupling of transmission line 10 tohousing 18.

The other end of transmission line 10 is coupled to the walls ofresonant cavity 14 by varactor diode 22. Varactor diode 22 is a voltagevariable capacitor which exhibits a capacitance when reversed biasedwith a DC potential. As the reverse bias across the varactor diodeincreases, the capacitance of the varactor diode decreases and as thereverse bias across the varactor diode decreases the capacitance of thevaractor diode increases.

In addition to transmission line 10, feed-through capacitor 20 andvaractor diode-22, resonant cavity 14 houses trimmer capacitors 24, 26and 28. In a preferred embodiment of the invention, trimmer capacitor 24comprises a physically adjustable metallic plate connected to one end oftransmission line 10 with the plate located adjacent the housing 18 toform capacitance coupling with the housing. Geometric variations in theplate configuration respective housing 18 adjust the magnitude of thecoupling. In a preferred embodiment, trimmer capacitors 26 and 28comprise metallic plates coupled to housing 18 and are physicallylocated along transmission line 10 to form capacitive coupling with line10.

In a similar manner, second transmission line 12 is connected toresonant cavity 16 in a half-wave mode with both ends capacitivelycoupled to the walls of second resonant cavity 16. Specifically,transmission line 12 is coupled to the walls of cavity 16 at one end bya fixed capacitor 30. Like fixed capacitor 20, capacitor 30 ispreferably of a feed-through variety. The other end of transmission line12 is coupled to the walls of resonant cavity 16 by varactor diode 32.Trimmer capacitors 34, 36 and 38 are located in cavity 16 alongtransmission line 12 in the same manner as trimmer capacitor 24, 26 and28.

As is well-known in the art, cavities 14 and 16 are aperture-coupledtogether by an opening 40 in the wall 42 dividing cavity 14 from cavity16. A loop of wire 44 extending from cavity 14 to cavity 16 throughopening 40 aids in the aperture coupling.

The preselector stage of the present invention further includes meansfor inductively coupling an unamplified radio frequency signal from theoutput of a UHF antenna 46 to the first resonant cavity 14 which meansfor inductive coupling comprises inductor 48 and resistor 50.

Inductor 48 is physically located inside cavity 14 adjacent transmissionline 10 and is coupled to antenna 46. Resistor 50 couples one side ofinductor 48 to housing 18 to bleed off undesirable static charge.

In accordance with the present invention, the preselector stage furtherincludes circuit means between transmission lines 10 and 12 forproviding the preselector stage with a substantially uniform bandwidthover the entire UHF frequency spectrum.

In a preferred embodiment, the circuit means comprises a "Bottom L"coupling including inductors 52, 54 and 56. Inductors 52 and 54 areconnected in series between the one end of the first transmission line10 coupled by capacitor 20 to the walls of cavity 14 and the one end ofthe second transmission line 12 coupled by capacitior 30 to the walls ofcavity 16. Inductor 56 is connected between inductors 52 and 54 andcapacitior 58 which, in turn, is connected to housing 18. Inductors 52,54 and 56 provide inductive coupling between said one end oftransmission lines 10 and 12 at the low frequency end of the UHFspectrum. As will be explained in more detail below, the inductivecoupling provided by inductors 52, 54 and 56 is frequency dependent inthat it is increasingly effective as the frequency of operation of thepreselector stage decreases.

To selectively tune the resonant frequency of the preselector stage, avariable DC reverse bias is applied to varactor diodes 22 and 32 toselectively alter the capacitance of those diodes. The DC voltage pathfor reverse biasing varactor diode 22 comprises the series combinationof resistor 60, decoupling resistor 62, inductor 56, inductor 52,feed-through capacitor 20 and transmission line 10. The DC voltage pathfor reverse biasing varactor diode 32 comprises the series combinationof resistor 60, decoupling resistor 62, inductor 56, inductor 54,feed-through capacitor 30 and transmission line 12. The amount ofreverse biasing across varactor diodes 22 and 32 is accordingly directlydependent on the magnitude of DC tuning voltage applied to resistor 60.Decoupling resistor 62 is employed to provide radio frequency decouplingbetween the first and second cavities 14 and 16 of the preselector andthe other circuits in the varactor tuner.

In operation of the preselector stage of the present invention,unamplified UHF radio frequency signals received by antenna 46 arecoupled to transmission line 10 by inductor 48. With capacitor 20coupling one end of transmission 10 to cavity 14 and varactor diode 22coupling the other end of line 10 to cavity 14, transmission line 10operates in a half-wave mode with peaks developed at each end of line 10and a null developed intermediate the ends of line 10. The frequency towhich line 10 is turned is dependent upon the capacitance of capacitor20 and varactor diode 22. Ignoring the distributed capacitance of line10, the resonant frequency of line 10 is proportional to the square rootof the sum of the two capacitances divided by the product of the twocapacitances. With the capacitance of capacitor 20 fixed, the resonantfrequency of line 10 therefore varies inversely with the capacitance ofvaractor diode 22. The higher the capacitance of varactor diode 22, thelower in the UHF spectrum line 10 is turned. Conversely, the lower thecapacitance of varactor diode 22, the higher in the UHF spectrum line 10is tuned. Since the capacitance of varactor diode 22 is inverselyproportional to the reverse bias across varactor diode 22, the frequencyto which line 10 is tuned increases with increased reverse bias anddecreases with decreased reverse bias.

With resistor 60 at 2.2K ohms, resistor 62 at 100 ohms, capacitor 20 at12 picofarads, and varactor diode 22 exhibiting about 2 picofarads witha tuning voltage of approximately 25 volts and about 18 picofarads witha tuning voltage of approximately 1.5 volts, line 10 can be made toselectively tune over the entire UHF spectrum from channel 14 to channel83.

In a similar manner, transmission line 12 is made to tune over theentire UHF spectrum with opening 40 providing aperture coupling betweenline 10 and line 12.

As mentioned above, as the capacitance of varactor diodes 22 and 32increases (decreasing the tuned frequency of the preselector stage) theQ or quality of the dioes 22 and 32 decreases. However, by employing thevaractor diodes 22 and 32 in a half-wave tuned circuit, the effect ofthe decreasing Q at the lower frequency operation is minimized. Thereason is as follows: at the low end of the frequency spectrum, thecapacitance of varactor diodes 22 and 32 greatly exceeds the fixedcapacitance of capacitors 20 and 30. As a consequence, the highimpedance point in lines 10 and 12 are developed at the lowercapacitance of capacitors 20 and 30 causing varactor diodes 22 and 32 toessentially appear as AC shorts to housing 18. Accordingly, at lowfrequency, the half-wave tuning of the present invention establishes, inessence, a substantially quarter-wave standing wave. Thus, even thoughthe Q of varactor diodes 22 and 32 decreases at the low end of thefrequency spectrum when the capacitance of diodes 22 and 32 ismaximized, the development of a high impedance point at the fixedcapacitors 20 and 32 decreases the contribution of the varactor diode Qto the overall circuit Q and increases the contribution of the fixedcapacitors Q to overall circuit Q, thereby maintaining high circuit Qthroughout the UHF TV band.

It should be noted that this feature would not be possible in a tunerusing quarter-wave tuned lines because one end of every quarter-waveline is permanently tied to the cavity walls.

By increasing the Q of the entire preselector circuit over the entireUHF spectrum, the resultant bandwidth of the preselector and, ingeneral, the overall signal strength and gain of the preselector aremaintained substantially uniform over the entire UHF spectrum.

However, although the employment of varactor diodes in the half-waveconfiguration of the present invention operates to increase the Q factorof the overall circuit, the Q of the circuit nevertheless tends to droptowards the low frequency end of the UHF spectrum when only aperturecoupling is employed between cavities 14 and 16.

To compensate for signal loss at the lower end of the UHF spectrum andin accordance with the present invention, inductors 52, 54 and 56augment the aperture coupling of opening 40 between lines 10 and 12 atthe lower end of the UHF spectrum. Specifically, at the low end of thefrequency spectrum the high impedance point of tuned lines 10 and 12transfers from the varactor diodes to the lower capacitance ofcapacitors 20 and 30 as explained above. With the high voltage potentialat capacitors 20 and 30, inductors 52, 54 and 56 become effectivecouplers of energy between line 10 and line 12. However, at the high endof the UHF spectrum, the capacitance of fixed capacitors 20 and 30greatly exceeds the reduced capacitance of varactor diode 22 and 32. Asa consequence, the high impedance point in line 10 and 12 moves to thelower capacitance of varactor diode 22 and 23 rendering fixed capacitors20 and 30 effective AC shorts to housing 18 and thereby diminishing thecoupling effect of inductors 52, 54 and 56. Thus, even though the Q ofthe tuned lines decreases at low frequencies, the available signal ismore efficiently coupled between the lines 10 and 12 thereby maintaininga constant and substantially uniform bandwidth across the entire UHFspectrum.

Returning to the drawing, an oscillator means is shown for generating anoscillator signal.

In accordance with the present invention, the oscillator meansincorporates a frequency control means comprising a halfwave tunedtransmission line 64 mounted in an oscillator resonant cavity 66. Oneend of tuned line 64 is coupled to the walls of resonant cavity 66 byfeed-through capacitor 68. The other end of tuned line 64 is coupled tothe walls of resonant cavity 66 by varactor diode 70. Trimmer capacitors72, 74 and 76 provide capacitive coupling respectively between the wallsof cavity 66 and transmission line 64.

To selectively tune the resonant frequency of line 64, a DC circuit isprovided by blocking coil or choke 78 from resistor 60 through capactior68 to line 64. Thus, as the tuning voltage increases, the reverse biasacross varactor diode 70 increases, decreasing the capacitance ofvaractor diode 70 but increasing the resonance frequency of tuned line64. Blocking coil 78 isolates radio frequency energy in cavity 66 fromother sections of the tuner.

In accordance with the present invention, the oscillator means furtherincludes a signal generating means comprising a transistor 80 coupled tosaid one end of tuned line 64. In a preferred embodiment of theinvention, transistor 80 is physically located within resonant cavity66. The emitter of transistor 80 is connected to the housing of cavity66 by resistor 82 and the emitter is further connected to said one endof tuned line 64 by capacitor 84.

The collector of transistor 80 is capacitively coupled to said one endof tuned line 64 by capacitor 86. In a preferred embodiment of theinvention, capacitor 86 is a feed-through capacitor physically coupledto the end of the feed-through capacitor 68 which extends into cavity66. B+ is provided to the collector of transistor 80 through resistor90.

The base of transistor 80 is biased by the voltage divider comprisingthe series combination of resistors 88 and 94 between the B+ source andhousing 18. A bias voltage is applied to the base of transistor 80 bymeans of feed-through capacitor 92.

In operation, transistor 80 sets up a standing half-wave signal in tunedline 64 whose frequency is dictated by the reverse bias across, andhence capacitance of, varactor diode 70. At high operating frequencies(low capacitance in varactor diode 70) the Q of the circuit is high.However, as the frequency decreases, the Q of the varactor diodedecreases and losses in the circuit increase. As explained above, theselosses are minimized by the employment of a half-wave configuration inthat at low frequencies, the high capacitance of varactor diode 70 incomparison to the capacitance of capacitor 68 renders varactor diode 70an effective AC short to housing 80 and shifts the high impedance pointof tuned line 64 to capacitor 68. This shift, as explained above,decreases the contribution of the varactor Q to the overall circuit Qand increases the contribution of the fixed capacitor Q to the overallcircuit Q, thereby maintaining a high overall circuit Q throughout theUHF spectrum.

However, at low frequencies, some decrease in Q is experienced due tothe the operation of varactor diode 70. This decreased Q is to a degreecompensated for by the placement of the signal generating means adjacentthe fixed capacitor end of tune line 64 which at low frequenciesexhibits a high impedance.

Returning to the drawing, in a preferred embodiment of the invention, aresistor 98 is shown located inside cavity 66 coupled at one end tohousing 18 with the other end passing through housing 18. Resistor 98thereby provides a sample oscillator output which may be utilized tocontrol and regulate the frequency of oscillation in tuned line 64 bymeans of a phase locked loop or counter circuit. Thus, resistor 98provides a constant impedance for such a circuit and can drive channelselection circuitry (not shown) in the receiver to control the tuningvoltage to varactors 22, 32 and 70.

In accordance with the present invention, mixer means is providedcoupled to heterodyne the oscillator signal with a select radiofrequency signal passed by the preselector stage to result in anintermediate frequency signal. Specifically, the preselector of thepresent invention results in a select radio frequency signal beingestablished in cavity 16 by the operation of double tuned half-wavetransmission lines 10 and 12. The oscillator signal is generated incavity 66 by the operation of tune line 64 as described above.

In a preferred embodiment of the invention, the mixer means comprisesmixer diode 100, feed-through capacitor 102, and inductor 104. Diode 100is inserted in a aperture stamped in the partition between cavities 16and 66 to accomplish mixing between the oscillator signal in cavity 66and the radio frequency signal in cavity 16 in a manner described in myU.S. Pat. No. 3,299,360 issued Jan. 17, 1967 and assigned to theassignee of the present invention. Specifically, a lead 106 is connectedfrom an internal wall of cavity 16 to diode 100 to provide couplingmeans for the RF signal in cavity 16 to diode 100.

In addition, a lead 108 runs from diode 100 into cavity 66, and backinto cavity 16 through feed-through capacitor 102. Lead 108 operates tocouple the oscillator signal in cavity 66 to diode 100. Lead 108connects to inductor 104 which, in turn, is coupled to outside cavity 16by lead 110. The inductively coupled radio frequency and oscillatorsignals are heterodyned in the diode 100 to provide a desired sum ordifference frequency signal of intermediate frequency at line 110.

The resultant signal on line 110 is selected to be at the intermediatefrequency for application to channel one of a VHF television tuner andthe output on line 110 may in fact be directly coupled to theintermedite freuency input of a conventional VHF tuner.

However, in accordance with the teachings of the present invention, apreferred embodiment is disclosed wherein the intermediate frequencysignal on line 110 is first amplified by a post gain controlintermediate frequency amplifier 112 to control the magnitude of theintermediate frequency signal from mixer diode 100 to the input of a VHFtuner. The intermediate frequency signal from line 110 is coupled toamplifier 112 by choke 114 after filtering of the signal by inductor 116between line 110 and housing 18. The use of a low noise, gain control IFamplifier stage between line 110 and the IF output of the tuner isparticularly desirable when the UHF tuner of the present invention isused in conjunction with a varactor VHF tuner. In such a case, theoutput of amplifier 112 may be readily coupled directly to the IF inputof a varactor VHF tuner. Furthermore, by employing a FET VHF inputamplifier and a FET in amplifier 112, the AGC signal used to control UHFtuner amplifier 112 and a VHF tuner amplifier may be derived from thesame source.

Thus, an output voltage is developed by the varactor UHF tuner of thepresent invention that is commensurate with VHF channel one inputwithout the need of UHF radio frequency preamplifier that is costlyand/or subject to producing cross-modulation and inter-modulationproducts. It is apparent that the varactor UHF tuner of the presentinvention attains combined economy and performance, heretofore notattainable by the prior art. Specifically, the present inventionprovides a comparatively low noise UHF tuner without the use of a radiofrequency preamplifier, which maintains nearly uniform bandwidththroughout the UHF spectrum and which is compatible with eithermechanical or varactor VHF tuners in which the VHF mixer is used as aUHF intermediate frequency amplifier.

While a particular embodiment of the present invention has been shownand described, it will of course be obvious to one skilled in the artthat certain advantages and modifications may be effected withoutdeparting from the spirit of the invention, and accordingly, it isintended that the scope of the invention not be determined by theforegoing example but only by the scope of the appended claims.

What is claimed is:
 1. A UHF varactor tuner comprising:preselector meansfor passing a select radio frequency signal, said preselector meansincluding a housing containing resonant cavities, a plurality oftransmission lines and a plurality of varactor diodes for providingvariable half-wave tuning of said transmission lines, said transmissionlines being contained in respective resonant cavities of said housingand being aperture intercoupled by aperture means in the walls of saidresonant cavities; oscillator means for generating an oscillator signal;mixer means coupled to heterodyne said oscillator signal with saidselect radio frequency to result in an intermediate frequency signal;and said preselector means further including frequency dependent circuitmeans coupled between said tuned transmission lines and operativeprimarily at one end of the UHF band of frequencies to augment saidaperture intercoupling and thereby provide said preselector means with asubstantially uniform bandwidth over the entire UHF band.
 2. The UHFvaractor tuner recited in claim 1 wherein said preselector means furtherincludes means for inductively coupling an unamplified radio frequencysignal from a UHF antenna output to said plurality of tuned transmissionlines.
 3. The UHF varactor tuner recited in claim 2 wherein said tunerfurther comprises low noise post intermediate frequency gain controlamplifier means coupled to control the magnitude of said intermediatefrequency signal from said mixer means for application of saidcontrolled intermediate frequency signal to an intermediate frequencysignal input of a VHF tuner.
 4. The UHF varactor tuner recited in claim3, including a plurality of fixed capacitors and wherein said tunedtransmission lines include first and second half-wave tuned transmissionlines and said preselector means also includes first and secondpreselector resonant cavities having conductive walls and respectivelyhousing said first and second tuned transmission lines, each tunedtranmission line being capacitively coupled to one end to a wall of itscavity by one of said varactor diodes, each tuned transmission linebeing further capacitively coupled at the other end thereof to anotherwall of its cavity by one of said fixed capacitors, said first andsecond preselector resonant cavities having aperture couplingtherebetween to form a double tuned half-wave preselector.
 5. The UHFvaractor tuner recited in claim 4 wherein said frequency dependentcircuit means comprises inductive coupling between said other ends ofsaid first and second half-wave tuned lines.
 6. The UHF varactor tunerrecited in claim 5 wherein said oscillator means includes a frequencycontrol means comprising a third half-wave tuned line, an oscillatorresonant cavity having conductive walls and an oscillator varactordiode, said third tuned line being capacitively coupled at one end tothe walls of said oscillator resonant cavity and said third tuned linebeing coupled at the other end to the walls of said oscillator resonantcavity by said oscillator varactor diode.
 7. The UHF varactor tunerrecited in claim 6 wherein said oscillator means further includes asignal generating means comprising a transistor coupled to said one endof said third half-wave tuned line.
 8. The UHF varactor tuner recited inclaim 7 wherein said mixer means comprises a diode mixer coupled betweensaid second preselector resonant cavity and said oscillator resonantcavity.
 9. A constant bandwidth UHF tuner comprising:first and secondresonant cavities, two transmission lines, two capacitors, and twovaractor diodes, said transmission lines being housed respectively insaid first and second resonant cavities in half-wave tuningconfigurations with one of said capacitors connected between a first endof each line and each respective cavity and with one of said varactordiodes connected between a second end of each line and each respectivecavity; variable voltage means for biasing said varactor diodes to tunesaid lines to a select radio frequency signal; radio frequency couplingmeans for introducing an unamplified ratio frequency signal to saidfirst cavity; aperture coupling means between said first and secondcavities; oscillator means for generating an oscillator signal; mixermeans coupled to heterodyne said oscillator signal with said selectradio frequency signal as passed by said resonant cavities to result inan intermediate frequency signal; and frequency dependent coupling meansinductively linking together said first end of said transmission line insaid first cavity and said first end of said transmission line in saidsecond cavity during operation of said tuner at the lower portions ofthe UHF frequency spectrum to provide said tuner with a substantiallyuniform bandwidth over the entire UHF frequency spectrum.
 10. The UHFtuner recited in claim 9 wherein said oscillator means comprises:a thirdoscillator resonant cavity, a third transmission, third capacitor and athird varactor diode, said oscillator line mounted in said oscillatorresonant cavity with said oscillator transmission line coupled at afirst end to said oscillator resonant cavity by said capacitor and saidoscillator transmission line coupled at a second end to said oscillatorresonant cavity by said third varactor diode; a signal generating meanscomprising a transistor coupled to said first end of said oscillatorline; and said variable voltage means biasing said third varactor diodeto tune said oscillator means to a select oscillator frequency signal.11. The UHF tuner recited in claim 10 wherein said mixer means comprisesa diode mixer coupled between said second resonant cavity and saidoscillator resonant cavity.
 12. The UHF tuner recited in claim 11wherein said tuner further comprises low noise post intermediatefrequency gain control amplifier means coupled to control the magnitudeof said intermediate frequency signal from said mixer means forapplication of said controlled intermediate frequency signal to anintermediate frequency signal input of a VHF tuner.